Most 50 amp water heater will come with 4500 watt elements. On a 240 volt circuit that is 4500 / 240 = 18.75 amps, so, no, that will not trip a 20 amp breakers, however, it will be operating the breaker nearly 95% of its 20 ampacity rating and most breakers are not rated for that. Typically, the circuit and breaker will be suited for use at up to 80% of it ampacity rating, meaning a 20 amp breaker will be suitable for 16 amps.

Another question becomes the size of the circuit, if the circuit is a #12 AWG copper, with a 20 ampacity rating, that also should be limited to 80% of its 20 ampacity rating, meaning the circuit conductors, *IF* #12 AWG copper, would be rated for 20 amps also.

*IF* the conductor size is #10 AWG, with a 30 amp rating, then the circuit becomes suitable for replacing the breaker with a 30 amp rated breaker to match the circuit rating.

Without knowing the circuit rating, the answer may be as simple as replacing the breaker, but maybe not - not if the circuit conductors are #12 AWG copper.

Another option, though, is found when reviewing the Rheem specification sheet: [Note: place cursor over link, right click, select ' Open in new window' (or a similar option)]: http://waterheating.rheem.com/content/r ... arrior.pdf , as Rheem offers those 50 gallon water heaters with 3500 watt elements. A 50 gallon water heater with 3500 watt elements can use a smaller circuit and breaker size, but they also will not provide the same amount of hot water nor the recovery time the occupants may be used to. Sure, they have a 50 gallon water heater, but what heats it ... a match? Just kidding.

Rheem's specification sheet shows the 3500 watt element 50 gallon water heater has a 10% less first hour rating, but has almost a 25% lower recovery rating.

*IF* the elements in that 50 gallon water heater were only 3500 watts, then on a 240 volt circuit you would have 3500 / 240 = 14.6 amps, which is within the 16 amps of the 80% rating of both a #12 AWG copper circuit, rating of 20 amps, and the 20 amp rated breaker.

When reviewing the Rheem installation instructions: [Note: place cursor over link, right click, select ' Open in new window' (or a similar option)]: http://waterheating.rheem.com/content/r ... orElec.pdf , page 9, has a chart for the different circuits and overcurrent protection specified for the different element sizes. -> The 3500 watt element is shown using a minimum 20 amp rated circuit (#12 AWG copper shown) for a 240 volt circuit with a 20 amp overcurrent device (breaker). -> The 4500 watt element is shown using a minimum 30 amp rated circuit (#10 AWG copper shown) for a 240 volt circuit with a 25 amp overcurrent device (breaker). At first glance one might think this is 'specifying' a 25 amp breaker maximum, however, the table with this information in it starts out stating "This guide recommends minimum branch circuit sizing and wire size based on the National Electrical Code.", which means that installing the allowable 30 amp rated breaker on the #10 AWG copper circuit is acceptable not only to the code, but to these manufacturer's installation instructions.

Now to the code which references how to install water heaters. Since you are in Florida, the reference is from the 2004 Florida Building Code, Residential, with 2006 Revisions.(underlining, bold, and red text are mine) - P2801.2 Installation. - - Water heaters shall be installed in accordance with this chapter and Chapters 20 and 24.

(from Chapter 20, see above reference to this chapter) - M2005.1 General. - - Water heaters shall be installed in accordance with the manufacturer’s installation instructions and the requirements of this code. Water heaters installed in an attic shall conform to the requirements of Section M1305.1.3. Gas-fired water heaters shall conform to the requirements in Chapter 24. Domestic electric water heaters shall conform to UL 174 or UL 1453. Commercial electric water heaters shall conform to UL 1453. Oiled-fired water heaters shall conform to UL 732.